Laboratory Tips and Tricks

The Importance of Sample Preparation in Modern Analytical Methods

Merlin K. L. Bicking, Ph.D., ACCTA, Inc.
Copyright 2002-2003 by ACCTA, Inc. All rights reserved. Unauthorized copying or distribution prohibited.

Modern sample analysis methods generally involve a similar sequence of procedures: planning, sampling, sample preparation, quantification, review, and reporting. While most laboratory workers would agree that all of these steps are of equal importance, in practice this isn't what usually happens. Advances in instrumentation, automation improvements, and increased regulatory scrutiny have lead most people to focus on the instrumental, review, and reporting parts of the method.

Sample preparation is often considered the "low tech" part of the method, and thus receives little attention. In some laboratories, the youngest, least experienced (and usually lowest paid) workers are assigned to the "Prep Lab." It is assumed that their inexperience will be less of a liability in this part of the organization.

The result is a flow chart that looks something like the graphic on the right, where the size of the text indicates the relative importance of the activity. You can see that little attention is given to the first three steps in the method, unless there is a problem, of course.

If you are familiar with a statistical concept known as "propagation of errors," you should be concerned about setting priorities in this way. Propagation of errors is a way or dealing with the real-world fact that every laboratory procedure generates some error. That is, every activity adds a little bit of error, and this error can't be eliminated, because it usually represents the operating limits of the equipment used (balances, pipets, volumetric flasks, etc.).

In a series of procedures, like the steps in an analytical method, the errors are carried along, or "propagated," to the next step. So, any additional errors that occur in the early steps, such as sample preparation, will still be there at the data review step. No amount of instrumental analysis, or re-analysis, can eliminate these errors. And if communication and documentation aren't perfect, you may not be able to even find the source of these errors!

A better way to look at sample analysis is shown in the flow chart to the right. Here we can see that the individual steps are all approximately equal in importance, and that sometimes it is necessary to repeat the analysis if the results aren't acceptable. If you look at analysis this way, it is easier to take a critical look at every step in the procedure, not just a few.

The Importance of Sample Preparation

Despite its "low tech" image, sample preparation is a critical part of every method. Although many procedures do not use sophisticated, and expensive, equipment, they still require considerable care in operation to ensure that reliable results are obtained.

One could make an argument that most sample preparation procedures require more chemistry "skill" to complete than the average GC/MS or HPLC analysis! Modern instruments have become "turn-key" systems, where little direct user intervention is required. The operator may have to check on supplies and accessories, but many maintenance, calibration, and check functions are now automated, and performed by the controlling software. Not so with sample preparation, where some sample/technique combinations will challenge the analyst to use all of his/her skills.

For example, all sample preparation procedures are based on sound chemical and physical principles. To successfully, and efficiently, use these procedures, you must understand these principles, and then apply your knowledge of them to the task at hand.

What "principles" are we talking about? Here's a list of the chemical and physical principles that are used in all (traditional and modern) sample preparation techniques, along with examples of techniques that use each principle. Of course, most techniques use multiple principles!

Do you understand all of these principles? Do you know how, and when, they are important in the sample preparation procedures you are performing? Can you use this information to improve efficiency and reduce error?

I hope you answered "yes" to all three questions. If not, then consider spending a little review time on the subject.

Good luck!

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